Capacitive micromachined acoustic transducer

ABSTRACT

A micromachined capacitive acoustic transducer including an electrode formed by a perforated plate and another electrode formed by a shallowly corrugated membrane anchored at one or more positions on the substrate which also supports the said perforated plate is described. Also disclosed includes: a fixed perforated plate; a movable shallowly corrugated membrane having holes to form acoustic filter to a certain frequency or a range of frequencies spaced from the perforated plate that is anchored in one or more location but loose at other locations; a support structure in the perforated plate maintaining the minimum separation between the membrane and the perforated plate near the perimeter.

CROSS REFERENCE

This application claims priority from and is a continuation applicationof a U.S. non-provisional patent application entitled “CapacitiveMicromachined Acoustic Transducer” filed on Apr. 13, 2005, having anapplication Ser. No. 10/907,706. That application is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The batch processing of micromachining has led to the emergence ofcapacitive micromachined transducers. These transducers offer a largerset of parameters for optimization of performance as well as ease offabrication and electronic integration. The fabrication and operation ofmicromachined transducers have been described in many publications andpatents. For example, U.S. Pat. Nos. 5,619,476, 5,870,351, 5,894,452 and6,493,288 describe the fabrication of capacitive-type ultrasonictransducers. U.S. Pat. Nos. 5,146,435; 5,452,268, and 6,870,937 alsodescribe micromachined capacitive transducers that are mainly used inthe audio range for sound pickups. In most structures, the movablediaphragm of a micromachined transducer is either supported by asubstrate or insulative supports such as silicon nitride, silicon oxideand polyamide. The supports engage the edge of membrane, and a voltageis applied between the substrate and a conductive film on the surface ofthe membrane causes the membrane to vibrate in response to the passingsound waves. In one particular case as described in the U.S. Pat. No.6,535,460, the diaphragm is suspended to allow it rest freely on thesupport rings.

Many micromachined condenser microphones use a similar membranestructure to that of large measurement microphones and studio recordingmicrophones. One common structure, shown in FIG. 1, consists of aconductive membrane 1 suspended over a conductive back-plate 5 that isperforated with acoustic holes 3. Sound detection is possible when theimpinging pressure wave vibrates the membrane 1, thus changing thecapacitance of the transducer 2. Under normal operation, the change incapacitance of the condenser microphone 2 is detected by measuring theoutput current 8 under constant-voltage bias. A pressure equalizationvent 4 in the back-chamber 7 prevents fluctuations in atmosphericpressure from collapsing the membrane 1 against the back-plate 5. Aprecision condenser microphone for measurement or calibrationapplications is capable of a uniform frequency response due to itsrelatively large air gap, on the order of 20 μm, behind the membrane.Silicon micromachined microphones, with membrane dimensions of 1-2 mm,require air gaps 6 on the order of a few microns to maintain adequatesensitivity due to the reduced motion that results from a smallermembrane. However, the reduced dimensions of the air gap magnify theeffects of squeeze-film damping, introducing frequency-dependentstiffness and loss. This creates undesirable variations in themechanical response with acoustic frequency. Furthermore, achieving alarge dynamic range and a high sensitivity can be conflicting goals,since large sound pressures may cause the membrane to collapse under itsvoltage bias. This traditional approach suffers from low sensitivity,especially at low frequencies.

In order to achieve wide bandwidth and high sensitivity, the developmentof high-performance diaphragm is of vital importance in the successfulrealization of condenser microphones. For most very thin diaphragms,however, large residual stress can lead to undesirable effects such aslow and irreproducible performances, if the processes cannot accuratelybe controlled. One technique for acquiring low-stress diaphragms is touse a sandwich structure, in which layers with compressive and tensilestress are combined. Another technique is to use the support structuresuch as outlined in the U.S. Pat. No. 6,847,090. U.S. Pat. No. 6,535,460also describes a structure that the membrane is freely suspended toallow it release the mechanical stress. Unfortunately, in this case, thefreely suspended membrane will have unstable sensitivity and unwantedlateral movement, resulting in the signal spew and posing thereliability issues.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a micromachinedacoustic transducer with micromachined capacitive elements for sensingacoustic waves.

It is a further object of the present invention to provide amicromachined acoustic transducer that comprises a perforated platesupported above a substrate.

It is another object of the present invention to provide a micromachinedacoustic transducer that has shallowly corrugated membrane that issuspended above a substrate.

It is a further object of the present invention to provide amicromachined acoustic transducer whose suspended and shallowlycorrugated membrane is anchored on the substrate at one or morelocations.

It is another object of the present invention to provide a micromachinedacoustic transducer that has wide bandwidth and high sensitivity, yetits operation is stable and reliable.

It is a further object of the present invention to provide amicromachined acoustic transducer that features the mechanism tosuppress the unwanted rolling noise at audio band.

It is another object of the present invention to provide a micromachinedacoustic transducer that has the shallowly corrugated structures thatfurther provide relatively stable sensitivity.

The foregoing and other objects of the invention are achieved by amicromachined acoustic transducer including a perforated plate supportedabove a substrate, a shallowly corrugated membrane that is suspendedabove the said substrate and the said suspended shallowly corrugatedmembrane is anchored on the said substrate at one or more locations.Each membrane supports a conductive electrode for movement therewith,whereby each perforated plate forms a capacitor with the conductiveelectrode. The capacitance of the said capacitor varies with movement ofthe membrane responsive to the acoustic wave. Conductive linesinterconnect said conductive electrodes to provide output signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects of the invention will be more clearlyunderstood from the following description when read in conjunction withthe accompanying drawings of which:

FIG. 1 is a cross-sectional view of a typical traditionallymicromachined microphone.

FIG. 2 shows a cross-sectional view of a micromachined acoustictransducer along the line A-A′ in FIG. 4 according to the preferredembodiment of the present invention.

FIG. 3 shows a cross-sectional view of a micromachined acoustictransducer along the line A-A′ in FIG. 4 according to another preferredembodiment of the present invention.

FIG. 4 shows a top plan view of a micromachined acoustic transduceraccording to the preferred embodiment of the present invention.

FIG. 5 shows a top plan view of a micromachined acoustic transduceraccording to another preferred embodiment of the present invention.

FIG. 6 shows a top plan view of a shallowly corrugated membraneaccording to another preferred embodiment of the present invention.

FIG. 7 shows an angled cross-sectional view of a micromachined acoustictransducer when in operation along the lines A-A′ and B-B′ in FIG. 4according to the preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We approach the problem of making a good and practical micromachinedacoustic transducer from a different perspective. Our stress releasingtechnique is to form corrugations in the membrane. The corrugatedmembrane is capable of releasing the built-in stress during theprocessing, thereby increasing the mechanical sensitivity of themembrane and reducing the irreproducibility. Compared with theconventional flat diaphragm, the shallowly corrugated membrane has anincreased sensitivity, especially for a high residual stress level.

Referring now to FIG. 2, this is a cross-sectional view of amicromachined acoustic transducer along the line A-A′ in FIG. 4according to the preferred embodiment of the present invention. Ashallowly corrugated membrane 11 is anchored at one end on the substrate12, and loose at the other end. The built-in stress in the membrane 11is released through corrugation 20 on the membrane 11. The built-instress in membrane 11 is further release through the loose end 22 onmembrane 11. A perforated plate 13 is supported on the substrate 12through anchor 21. Perforation holes 14 are regular distributed on theplate 13 to allow the passage of acoustic wave. An air gap 25 is formedbetween the perforated plate 13 and membrane 11. On the back ofperforated plate 13, electrode 15 forms a capacitor with the membrane11. When acoustic wave passes through the perforation holes 14, themembrane 11 will vibrate in response to the acoustic wave, therebygenerating changing capacitance in the capacitor formed by perforatedplate 13 and membrane 11. The electrode 15 can also be placed on top ofthe perforated plate 13, as shown in FIG. 3.

Holes 17 and 18 are formed by the photolithography process. The size ofthese holes and their relative positions are chosen such that they willform a low-pass filter that allows the passage of slowly varying ambientpressure change across the stack of membrane 11, air gap 25 andperforated plate 13. But it will stop the leakage of acoustic signal atdesired frequency. Holes 16 are also formed in the photolithographyprocess to help release the sacrificial material under membrane end 22.On perforated plate 13, there are a series of spacers 19. They protectsthe membrane 111 from collapsing into the perforated plate 13 while inoperation in which the membrane 11 will be pulled towards the perforatedplate 13 when applied with bias voltage across them. Spacers 19 arediscontinuous, but they can also be made continuous to form a ring typestructure.

Referring to FIGS. 4 and 5, membrane 11 is anchored on to the substrate12 at one or more positions. If it is anchored at the positions shown inFIG. 4, it will essentially form a turning fork structure. In anotherpreferred embodiment according to this invention, the membrane 11 isanchored to the substrate 12 at many locations, as shown in FIG. 5.Contact pads 24 are used to wire the external circuit to the saidmicromachined acoustic transducer.

FIG. 6 shows the top view of the membrane 11. At the center region ofthe membrane 11, there may exist some holes 23. These holes are definedto form acoustic filter to a certain frequency or a range offrequencies. The size of holes 23 may be uniform, non-uniform, or aspread according to the filtering needs. The length of corrugation 20also varies depends on the desired sensitivity requirement. When asuitable length of corrugation 20 is achieved, the membrane 11 may beresting on the spacers 19 under bias voltage applied across thecapacitor formed by membrane 11 and perforated plate 13. In this case,the bending rigidity of the membrane 11 may be largely reduced becausethe active region of the membrane 11 will be those bounded by thespacers 19. This essentially reduces “equivalent thickness” of themembrane 11 due to corrugation 20.

For a condenser microphone, the measured sensitivity can be expressedas:${Sensitivity} \propto {\frac{a^{2}}{8 \cdot \sigma \cdot h} \cdot \frac{V_{b}}{d - d_{V_{b}}}}$

Where a is the radius of membrane 11, σ is the residual stress inmembrane 11, h the thickness of membrane 11, d the air gap 25 distance,and d_(Vb) is the change of air gap 25 distance under bias voltageV_(b). When the bias voltage V_(b) increases, the sensitivity of themicrophone also increases. In most of the applications, this is not thedesired results. And therefore, the alternative is to increase thebending stress in membrane 11 when the bias voltage increases. Referringto FIG. 7, which is the cross sectional view of line BB′ in FIG. 5. Whenin operation, the membrane 11 will be pulled to bend towards theperforated plate 13. Increasing the bias voltage will result in themembrane 11 bending further. Since the membrane 11 is anchored, furtherbending of membrane 11 from its desired operation state will result inthe increase of bending stress. This will then compensate thesensitivity increase due to the rise of bias voltage.

The foregoing descriptions of specific embodiments of the presentinvention are presented for the purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed; obviously many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A micromachined miniature acoustic transducer including a capacitorformed by a perforated plate and a membrane; a perforated plate havingwell defined perforation holes that serves as one electrode of the saidcapacitor; a shallowly corrugated membrane that is anchored at one ormore locations to form another electrode of the said capacitor. Thelength of corrugation for the said membrane is predetermined based onthe desired sensitivity requirement; a substrate on which the saidmembrane and the said perforated plate are anchored; and the saidmembrane having an anchoring structure that forms a tuning fork with thesaid perforated plate.